Photolithography is commonly used during formation of integrated circuits on semiconductor wafers. More specifically, a form of radiant energy (such as, for example, ultraviolet light) is passed through a radiation-patterning tool and onto a semiconductor wafer. The radiation-patterning tool can be, for example, a photomask or a reticle, with the term “photomask” traditionally being understood to refer to masks which define a pattern for an entirety of a wafer, and the term “reticle” traditionally being understood to refer to a patterning tool which defines a pattern for only a portion of a wafer. However, the terms “photomask” (or more generally “mask”) and “reticle” are frequently used interchangeably in modern parlance, so that either term can refer to a radiation-patterning tool that encompasses either a portion or an entirety of a wafer.
Radiation-patterning tools contain light restrictive regions (for example, totally opaque or attenuated/half-toned regions) and light transmissive regions (for example, totally transparent regions) formed in a desired pattern. A grating pattern, for example, can be used to define parallel-spaced conductive lines on a semiconductor wafer. The wafer is provided with a layer of photosensitive resist material commonly referred to as photoresist. Radiation passes through the radiation-patterning tool onto the layer of photoresist and transfers the mask pattern to the photoresist. The photoresist is then developed to remove either the exposed portions of photoresist for a positive photoresist or the unexposed portions of the photoresist for a negative photoresist. The remaining patterned photoresist can then be used as a mask on the wafer during a subsequent semiconductor fabrication step, such as, for example, ion implantation or etching relative to materials on the wafer proximate the photoresist.
Advances in semiconductor integrated circuit performance have typically been accompanied by a simultaneous decrease in integrated circuit device dimensions and a decrease in the dimensions of conductive elements which connect those integrated circuit devices. The demand for ever smaller integrated circuit devices brings with it demands for ever-decreasing dimensions of structural elements, and ever-increasing requirements for precision and accuracy in radiation-patterning with reticles and photomasks.
A prior art process of forming a pattern in a radiation-sensitive material is described diagrammatically with reference to FIG. 1. Specifically, FIG. 1 illustrates a fragment 10 of a reticle and a fragment 30 of a substrate having a radiation-sensitive material 32 thereover.
Reticle fragment 10 comprises a material 12 which is relatively opaque to radiation utilized for patterning radiation-sensitive material 32, and further comprises a feature 14 extending through the opaque material 12. Feature 14 is illustrated as a square feature, and such can be ultimately utilized to pattern a substantially circular structure 34 into radiation-sensitive material 32. Feature 14 can be referred to as “corresponding” to structure 34, in that actinic radiation passing through feature 14 prints structure 34 into radiation-sensitive material 32.
An outrigger pattern 16 extends around feature 14. Outrigger pattern 16 does not correspond to any structure printed into radiation-sensitive material 32, but rather is utilized to modify radiation passing through feature 14. Outrigger 16 aids in printing structure 34 in a desired shape. For instance, if structure 34 is desired to be a circle, it can be difficult to print the structure utilizing any particular shape of feature 14 alone, but a combination of a feature and an outrigger can create printed structures which closely approximate circles. Thus outrigger 16 modifies the printed structure corresponding to feature 14 relative to a structure which would be printed under identical conditions in the absence of outrigger 16.
Outrigger 16 is, as shown, a pattern formed to extend within the opaque material 12. The pattern of outrigger 16 can be entirely transparent to the radiation utilized for printing structure 34, or can be only partially transparent to such radiation. It is not uncommon for outrigger 16 to have a different transparency relative to the radiation than does feature 14, with feature 14 typically being entirely, or at least nearly entirely transparent to the radiation, and outrigger 16 being typically less transparent to the radiation. Also, outrigger 16 can induce a different shift in phase to radiation passing therethrough than does feature 14.
In interpreting this disclosure and the claims that follow, the term “outrigger” is utilized to describe an element associated with a reticle and utilized to assist in the printing of a structure in radiation-sensitive material. An outrigger is proximate a feature associated with the reticle which corresponds to the structure printed in the radiation-sensitive material. However, an outrigger is defined as an element that is spaced from the feature, rather than being in direct contact with the feature. In contrast, a “rim” (or “rim shifter”) is an element formed in a reticle and having a function similar to that of an outrigger, but differing from an outrigger in that the rim actually contacts an edge of the feature. Both rim shifters and outriggers are elements which modify the printed structure corresponding to features of a reticle relative to the structures which would be printed under identical conditions in the absence of either the rim shifters or outrigger. Also, both rim shifters and outriggers are configured to modify printed structures corresponding to features on a reticle other than the rim shifters and outriggers, rather than to directly correspond to any printed structures.
In the description that follows, the structures associated with fragment 10 of the radiation-patterning tool can be considered to correspond to a radiation-patterning tool domain, and the structures associated with substrate 30 can be considered to correspond to a printed domain.
FIG. 2 illustrates aspects of a prior art printing process in which two structures intended to be similar to the structure 34 of FIG. 1 are attempted to be printed very close together. The phrase “very close together” indicates that the structures are to have an edge-to-edge distance from one another of less than or equal to a wavelength of the actinic radiation utilized in the printing process. The actinic radiation can correspond to a distribution of wavelengths, and in such aspects the wavelength utilized to determine if the printed features are “very close together” can be any of the actinic wavelengths of the distribution, including, for example, a predominant wavelength of the distribution, a central wavelength of the distribution (which may or may not also be the predominant wavelength), a minimum wavelength of the distribution, a maximum wavelength of the distribution, etc. The structures 34 and 36 can have minimum widths of about the same dimension as a wavelength of the actinic radiation utilized for forming the structures, or smaller. In some applications, the separation of the structures can be a distance comparable to the minimum widths of the structures. Imaging of features which are very close together can be particularly difficult with on-axis illumination, and yet on-axis illumination is typically desired for isolated features (i.e., features which are not very close together). One aspect of the invention described and claimed herein is that it can allow on-axis illumination to be utilized for patterning both isolated and densely packed features.
FIG. 2 shows a fragment 50 of a radiation-patterning tool, and a fragment 70 of a substrate (such as, for example, a semiconductor wafer) having a radiation-sensitive material 72 thereover. Radiation-patterning tool 50 comprises a relatively opaque material 52 having a pair of feature patterns 54 and 56 formed therein, and an outrigger pattern 58 formed around the feature patterns. Feature patterns 54, 56 are utilized to form structures 34 and 36 within radiation-sensitive material 72 during a printing process. A problem than occurs due to the closeness of structures 34 and 36 to one another is that the printed structures are not separate and discrete from one another. Instead, a bridge 38 forms between the structures. A further problem that can occur when printing structures very close together can be a poor depth of focus.
It would be desirable to develop radiation-patterning tool configurations suitable for printing structures which are very close to one another, but yet separate and discrete from one another.